3690
Inorg. Chem. 1992, 31, 3690-3692
Fej-Triangle Opening and Closing by a Single Two-Electron Process in the Bicapped Triiron Clusters F ~ ~ ( C O ) ~ ( C ~ ~ -( P MM L L= )C~p F e ( C 0 ) 2 , CpMn(CO)d Yoshihiro Koide, Maria T. Bautista, Peter S. White, and Cynthia K. Schauer’ Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290 Received December 20, 1991 The structure of a metal cluster can often be rationalized by the total number of valence electrons it p0ssesses.l The pathways for interconversion between structural types can be studied by electrochemical manipulation of the number of valence electrons.2 Upon reduction of a saturated multimetal cluster, the M-Mbonded framework can undergo distortions along a variety of coordinates to minimize the antibonding interaction^,^ one possibility of which is a localized distortion resulting in cleavage of a single M-M b ~ n danalogous ~ ~ , ~ to what occurs in the case of a metal-metal-bonded dinuclear complex.5 We report here the first definitive example of a multimetal cluster that undergoes localized bond cleavage and bond formation by a quasi-reversible, single 2-e- process relating the closed and open6Fe3cluster frames in complexes of formulation Fe3(C0)9(p3-PML,)2 (1, ML, = c ~ F e ( C 0 )2, ~ ;ML, = CpMn(CO)2). For the case of 2, the reduction reaction to 22- was characterized by an X-ray structure determination of the 2-e- reduced product as well as an infrared spectroelectrochemical experiment that permitted observation of the 1-e- reduced intermediate. Thecluster Fe3(C0)9[p3-PFe(C0)2Cp]2 (1)’possesses an openFe3(p3-P)2core (with two Fe-Fe bonds) and can be viewed as a 50-electron metallophosphine ligand, [ Fe3(C0)9(p3-P)2]2-, that is coordinated to two C P F ~ ( C O )groups. ~+ The Fe3(p3-P)2cluster core in 1 is isostructural with that in the bicapped phosphinidene ( I ) See, for example: (a) Wade, K. Adu. Inorg. Chem. Radiochem. 1976, 18, 1. (b) Lauher, J. W. J . Am. Chem. SOC.1978, 100, 5305. (c) Mingos, D. M. P. Acc. Chem. Res. 1984, 17, 31 1. (2) For reviews, see: (a) Geiger, W. E.; Connelly, N. G. Adu. Organomer. Chem. 1985, 24, 87. (b) Geiger, W. E. Prog. Inorg. Chem. 1985, 33, 275. (c) Lemoine, P. Coord. Chem. Reu. 1988,83, 169. (d) Drake, S. R. Polyhedron 1990,9,455. (e) Lemoine, P. Coord. Chem. Reu. 1982, 47, 5 5 . (3) In the 49-electron bicapped tricobalt clusters of formulation [Co3(q5Cp)3(p,-L)2]+, examples ofuniformelongationof the threeCc-Coedges, elongation of a single Cc-Co edge, and elongation of two Co-Co edges with shortening of the third have been observed. (a) Pulliam, C. R.; Thoden, J. B.; Stacy, A. M.; Spencer, B.; Englert, M. H.; Dahl, L. F. J . Am. Chem. SOC.1991, 113, 7398. (b) Olson, W. L.; Dahl, L. F. J. Am. Chem. SOC.1986,108,7657. (c) Bedard, R. L.; Dahl, L. F.J. Am. Chem. SOC.1986, 108, 5942. (4) There are very few clusters where an overall 2-e- process has been definitively associated with bond formation or bond breaking localized onasingle M-M edge. Seeref 3aand: (a) Lockemeyer, J. R.; Rauchfuss, T. B.; Rheingold,A. L. J . Am. Chem.Soc.1989, I l l , 5733. (b) Whitmire, K. H.; Shieh, M.; Lagrone, C. B.; Robinson, B. H.; Churchill, M. R.; Fettinger, J. C.; See, R. F. Inorg. Chem. 1987, 26, 2198. ( 5 ) Reversible M-M bond cleavage occurs in many examples of bridged dinuclear complexes by either two sequential 1-e- steps or apparent 2-eprocesses (see reviews in ref 2). For well-characterized examples of single 2-e- processes associated with M-M bond cleavage in dinuclear complexessee: (a) Collman, J. P.; Rothrock, R. K.; Finke, R. G.; Moore, E. J.; Rose-Munch, F. Inorg. Chem. 1982, 21, 146. (b) Connelly, N. G.;Lucy, A. R.; Payne, J. D.; Galas, A. M. R.;Geiger, W. E. J. Chem. Soc.,Dalron Trans. 1983,1879. Freeman, M. J.;Orpen, A. G.;Connelly, N. G.;Manners, I.; Raven, S. J. J. Chem. SOC.,Dalron Trans. 1985, 2283. (c) Feng, D.; Schultz, F. A. Inorg. Chem. 1988, 27, 2144. Schultz, F. A. J . Electroanal. Chem. Fernandes, J. B.; Zhang, L. Q.; InrerfacialElectrochem. 1991,297, 145. (d) Moran, M.; Cuadrado, I.; Masaguer, J. R.; Losada, J. J . Chem. SOC.,Dalron Trans. 1988, 833. (e) Moulton, R.; Weidman, T. W.; Vollhardt, K. P. C.; Bard, A. J. Inorg. Chem. 1986,25, 1846. (0Gaudiello, J. G.; Wright, T. C.; Jones, R. A.; Bard, A. J. J . Am. Chem. SOC.1985, 107, 888. (g) Mann, K. R.; Rhodes, M. R. Inorg. Chem. 1984, 23, 2053. Hill, M. G . ;Mann, K. R. Inorg. Chem. 1991, 30, 1431. (6) In this paper the simplified termsopen and closed will be used to represent nido and closo, respectively. (7) Bautista, M. T.; White, P. S.;Schauer, C. K. J . Am. Chem. SOC.1991, 113, 8963.
0020-1669/92/1331-3690$03.00/0
I
1
clusters F ~ ~ ( C O ) ~ ( P ~ (I).* -PRA ) ~related cluster, Fe3(C0)9[p3-PMnCp(CO)2]2(2),prepared by Huttner and co-workers,9 I
2
has a neutral closed-Fe3(p3-P)2 core and can be viewed as a 48electron metallophosphine ligand, Fe3(CO)g(p3-P)2, that is coordinated to two neutral CpMn(CO)* groups. This study was initiated to explore the relationship between clusters with closed and open triangular cores. The cyclic voltammograms of 1 and 2 are shown in Figure 1;lO the results for 2 will be discussed first. The 48-electron closed cluster 2 shows an electrochemically quasi-reversible reduction waveinCH3CNsolutionatEl/2= -0.861 VvsAg/O.l MAgNO3. The separation between the cathodic and anodic peaks, At?,,is 36 mV at scan speeds of 25 mV s-l or less,” suggesting that more than one electron is involved in the electrochemical process.12 The bulk reduction13 of 2 at -1.0 V proceeds with napp= 2.0 faradays mol-], and the intensely brown-purple solution of 2 is replaced by a red-orange solution of 22- that is very similar in color to l.I4 The C O stretching frequencies of the two-electron reduced product are shifted by approximately 60 cm-i to lower (8) Cook, S.L.; Evans, J.; Gray, L. R.; Webster, M. J . Organomef. Chem. 1982, 236, 367. (9) Lang, H.; Huttner, G.;Zsolani, L.; Mohr, G.;Sigwarth, B.; Weber, U.; Orama, 0.; Jibril, I. J . Organomet. Chem. 1986, 304, 157. (IO) Cyclic voltammograms were recorded in a single-compartment airtight three-electrode cell under nitrogen at room temperature. All potentials reported in thispaperareconverted totheAg/AgN03(0.1 MinCH3CN) reference using the potential for the CpZFe/(CpZFe)+ standard (E112 = 0.042 V). In acetonitrile solution, a Ag/AgNO, reference electrode, which was separated from the analyte solution by placement in a vycortipped compartment, was employed. Electrochemical measurements in CH2C12 solution employed a Ag/AgCI reference electrode immersed in a 3 M aqueous sodium chloride solution in a separate vycor-tipped compartment. ( 1 1 ) Experiments to obtain peak-to-peak separations were performed in acetonitrile solution utilizing a silver wire reference electrode. All cyclic voltammetry data were corrected for uncompensated resistance by the positive-feedback iR compensation method or by correcting the AE, values using the peak-to-peak separation (AE, (mv)) and anodic peak current (ipa(+A)) observed for the Cp2Fe/(Cp2Fe)+ standard couple. (12) Polcyn, D. S.; Shain, I. Anal. Chem. 1966, 38, 370. (1 3) Bulk electrolyses were performed at a Pt-mesh electrode in tetrahydrofuran solution with an electrolyte concentration ([n-Bu4N] [ B S ] ) of 0.1 M and a sample concentration of 5 mM. (14) The UV-vis spectra are available as supplementary material.
0 1992 American Chemical Society
Inorganic Chemistry, Vol. 31, No. 18, 1992 3691
Communications 100
80
-8 8 E
60
Y
IhIIII 1.0
0.5
V 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 I I I I I I l I I I I
0.0
-0.5
-1.0
-1.5
c
m .-
-2.5 ( V VS. Ag/O.l M AgNO3 ) -2.0
Figure 1. (a) Cyclic voltammograms (scan speed = 100 mV s-I) of a ca. 1 m M solution of F ~ x ( C O ) ~ [ ~ ~ - P F ~ ( (1) C Oin) ~0.1 C ~M] ~ [n-Bu4N] [ B F I ] / C H ~ C Iat~ a platinum working electrode. (b) Cyclic voltammogram (scan speed = 50 mV s-I) of a ca. 1 m M solution of F~~(CO)~[~~-PMII(CO)~C~]~ (2) in 0.1 M [n-Bu4N] [BFd]/CHjCN a t a glassy carbon working electrode.
energy.15 The dianion 22- is quite stable, and reoxidation back to 2 passes charge corresponding to napp= 2.0 faradays mol-]. The reduction of 2 is one of a growing number of examples of apparent 2-e- behavior in dinuclear complexes and metal ~ 1 ~ ~ t e r ~ . ~As~opposed ~ ~ . ~ to~ the . 5 unlikely . ~ 6 simultaneousaddition of two electrons, an apparent 2-e- reduction wave is typically attributed to two consecutive 1-e- reductions where addition of the second electron occurs with nearly equal or greater facility than addition of the first.17 The difference between the l-epotentials is reflected in hE, for the 2-e- wave1' and in the value of the disproportionationconstant,,si& governingthe equilibrium shown in eq 1 for the 1-e- reduced species, 2-. For cases where Kdilip
2 2- + 22-
+- 2
(1)
the 1-e- reduced product is observable, measurement of &isp enables extractionof the individualone-electronpotentials.ls The strong carbonyl stretches in the infrared spectrum for 2 provide a sensitive spectroscopic tool for characterization of the electrochemical reduction reaction. Electrolysis of 2 in an infraredtransparent electrochemical cell reveals the strongest CO stretch for the mononegatively charged cluster radical at 1996 cm-I (Figure 2), intermediate in energy between the stretches for 2 (15) IR (YCO, THF, cm-1) for 22-: 201 1 (w). 1971 (vs), 1952 (s), 1931 (m), 1844 (w). (16) (a) Tulyathan, B.; Geiger, W. E. J. Am. Chem. SOC.1985, 107, 5960.
(b) Barley, M. H.; Drake, S.R.; Johnson, B. F. G.; Lewis, J. J . Chem. SOC.,Chem. Commun. 1987, 1657. (c) van der Linden, J. G. M.; Paulissen, M. L. H.; Schmitz, J. E. J. J . Am. Chem. SOC.1983, 105, 1903. (d) Amarasekera, J.; Rauchfuss, T. B.; Wilson, S.R. J . Chem. SOC.,Chem. Commun. 1989, 14. (e) Hinkelmann, K.; Heinze, J.; Schacht, H. T.; Field, J. S.; Vahrenkamp, H. J. Am. Chem. SOC.1989, 111, 5078. ( f ) Nembra, G.; Lemoine, P.; Gross, M.; Braunstein, P.; de Mtric de Bellefon, C.; Ries, M. Electrochim. Acra 1986, 31, 1205. (9) Gourdon, A.; Jeannin, Y. J . Organomet. Chem. 1985, 290, 199. (h) Nemra, G.; Lemoine, P.; Braunstein, P.; de Meric de Bellefon, C.; Ries, M. J. Organomet. Chem. 1986, 304, 245. (17) (a) Richardson, D. E.; Taube, H. Inorg. Chem. 1981, 20, 1278. (b) Myers, R. L.; Shain, I. Anal. Chem. 1969, 41, 980. (c) Hinkelmann, K.;Heinze, J. Ber. Bunsen-Ges. Phys. Chem. 1987, 91, 243. (18) The observed Eli2 for an apparent 2-e- wave for the reaction defined in eq 2 is an average of the potentials for the individual I-e- couples: E112 = F = (PI+ F 2 ) / 2 . Equation 3 relates thedisproportionation constant to the difference between the I-e- potentials. These two expressions together enable extraction of Eo', and Eo'2.
(3)
40
2
c
.. .. ..., .... ..,. .... .. .. ..
t
.:
2-
e 222
20
" '
0 2100
2000
.1
1900
1800
cm
Figure 2. Plot of the infrared spectral changes as a function of time during the bulk electrolysis of Fe3(C0)&3-PMn(CO)zCp]2 (2) in an infrared-transparent electrochemical cell (0.1 M [n-BudN] [BFp]/tetra hydrofuran).
and 22- as expected.I9 From analysis of the infrared data taken over the course of the electrolysis, the value &isp = 10 is estimated,20 implying that the second reduction occurs at a potential approximately 60 mV more positive than the first.I7 This value of AE1p predicts a value of AE, equal to 34 mV,I7 which is in good agreement with the experimentally observed value of 36 mV. Typically,the potentials for sequential addition of two electrons are well separated due to electrostatic repulsion between the first and second electrons. The fact that addition of the second electron to 2 is more favorable than addition of the first presumably results from a structural change that is coupled with the reductiona21A chemical reduction of 2 was performed,22and a single-crystal X-ray diffraction study of [(Ph3P)~N]2[2~-]~~ (Figure 3) clearly shows that the closed Fe3 frame in 2 has been transformed to an open Fe3 frame isostructural with 1 (eq 4). The bonding Fe-Fe distances (2.660 (9) and 2.660 (9) A) are much shorter than the (19) IR
(YCO, THF, cm-I, by spectral subtraction) for 2-: 2040 (w), 1996 (vs), 1967 (m), 1958 (w sh), 1946 (w sh), 1930 (w sh), 1885 (w).. A room-temperature EPR spectrum of 2- in tetrahydrofuran solution showed a broad singlet at g = 2.014. No well-resolved phosphorus coupling was observed (see supplementary material). The EPR sample was prepared by partial reduction (-70%) of a 7.6 mM solution of 2 by stirring over 4% Na/Hg amalgam. (20) Because the spectra of 2.2-, and 22- severely overlap, it is necessary to estimate the concentration of 2- by spectral subtraction. (21) Large heterogeneous electron-transfer rate constants (0.2cm s-1) were found for a series of dimers that exhibit significant structural changes as a result of the electron transfer. Several explanations are offered for this fact, including the possibility that the electron transfer is not kinetically coupled to the structural change. Gennett, T.; Geiger, W. E.; Willett, B.; Anson, F. J. Electroanal. Chem. Interfacial Electrochem. 1987, 222, 151. (22) A tetrahydrofuran solution of 2 was reduced by titration with a Na/ benzophenone solution in tetrahydrofuran while the IR spectrum of the solution was periodically monitored. A color change from brown-purple to red-orange accompanies the reduction. When the reduction was complete, the solution was cannulated into a flask containing 2 equiv of [(Ph,P)lN]CI and stirred overnight. The solvent was removed under vacuum, the residue was washed with MeOH and then dissolved in CH2CI2. Layering the CH2CI2 solution with Et20 yielded crystalline [PPNI2[22-]. A single 31P NMR resonance attributable to 22- was observed at 6 590 ppm in CHzCl2 solution.
3692 Inorganic Chemisfry. Vol. 31. No. 18, 1992
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Communications
slossd
open
two electrons in the wave for 1. Although the instability of the n-3. Ball and stick diagramof the metal-phosphoruscare from the O ~ ~ Cproduct ~ ) ~ ~1'+ ~ ~precludes . ~ C Ha~detailed C I ~ .characterization of the crystal structure of [ P P N ~ Z [ F ~ ~ ~ C O ~ ~ ( ~ ~ - P M ~ ~ Coxidized Selected distances (A): FeI-Fe3 = 2.660 (9). Fe2-Fe3 = 2.660 (9). disproportionation equilibrium, the U p value observed in the Fel-Fe2 = 3.532 (10). p14'2 = 2.768 (17). Fel-PI = 2.301 (14). cyc~icvo~~mmogramsugges~i~iss~ight~y~essthermodynamica~~y Fcl-P2=2.247(14).FeZ-PI =2.232(14),FeZ-P2=2.251 (14).Fe3favorable to remove the second electron than the first.26 On the PI = 2.239 (12). Fe3-P2 = 2.204 (14). Mnl-PI = 2.245 (14). MnZ-PZ basis of the analogy of the behavior for 2 reported above, the = 2.282 (14). unstable oxidized product, l'+, is suggested to have a closed structure like 2. nonbonding interaction (3.532 (IO) A). Extended H k k e l Thephenylphosphinidenecluster Fej(C0)9(pj-PPh)2,2' which calculationsz' were performed on 2 to examine the character of is isostructural with 1, displays very different electrochemical the orbitals involved in the electrochemistry. The picture that behavior. Twochemicallyrevenible I-e-reductions (E,,, =-1.18 emerges from these calculations is quite similar to that found in and-1.67 V) andanirreversibleca. three-electronoxidation (E; Fenske-Hall calculations performed on the triangular cluster =0.99V) areobserved. Consistent with theview that thecluster F e j ( C 0 ) p ( ~ ~ - Sin) ~both a symmetrical geometry with three core in 1isformallydinegativelycharged (seeabove). theoxidation equivalent Fe-Fe bonds and a distorted geometry with one of 1 is more facile and the reduction of 1 more difficult than for elongated Fe-Fe edge25 The LUMO is an in-plane antibonding FedC0)9(w-PPh)~. a'symmetry orbital (in Cjhsymmetry)of primarily d,,character. In the cases of 1 and 2, the opening and closing of the Fej Upon distortion of the structure by elongation of a single Fe-Fe triangleoccurs by an apparent 2-e-process. Dahlandco-workers edge, a more stable localized Fe-Fe antibonding orbital is recently reported conversion ofthe open cluster [Coj(CrH4Me),produced by mixing the a' LUMO with the next higher lying (pj-S)~](3) to the closed cluster, 3'+, in two I-e- steps separated orbital of a' symmetry (derived from an e' set of orbitals in Cjh by 0.8 V.'. The structures of 3 and each of the oxidized products symmetry based on d, orbitals). The energetic consequences of show that thesucassiveoxidationsareaccompaniedby shortening the Fe-Fe bond elongation and other coupled structural changes of the Cc-Co nonbonding distance (3.19 A for 3 , 2 3 7 A for 3+, that take place along the two-step electron-transfer reaction and 2.52 A for the equilateral triangle in 32+),similar to the net coordinate must result in the second electron transfer being structural changes involved in interconverting 22- and 2. In a thermodynamicallyfavored overthe first. despite the unfavorable related system, the open cluster Ruj(arene)j(pj-S)2is converted electrostatic considerations. tocl~sed-[Ru~(arene)~(p~-S)~]'+ by two 1-e- wavesseparated by The 50-electron open cluster 1 is electrochemically less wellonly 0.14 V." Clearly, the influenceof very similar net structural behaved than 2 (see Figure I ) . A chemically and electrochemchanges on the thermodynamics of the first and second electron ically quasi-reversible oxidufion wave is observed in CH2C12 transfer in a 2-e- process is quite variable. We are exploring the solution at Et,z = 0.160 V (Up = 48 mV at scan speeds of 50 electrochemistry of other clusters related to 1 and 2 to see if the mV S-Ior less) presumably the reverse of the process observed factors favoring 2-e- behavior in this series of FejP2clusters can in the reduction wave for 2 (see eq 4). The lack of complete be clearly determined. chemical reversibility is evidenced by the peak on the reverse scan a1 E, = -0.4 V. which is more pronounced at slower scan Acknowledgment. Professor W. E. Geiger, Jr. (University of speeds and in the more coordinating solvents tetrahydrofuran Vermont) and Professor R. W. Murray (UNC-CH) are acand acetonitrile. A cyclic voltammogram was obtained of a knowledged for enlightening discussions. Partial financialsupport solution containing equimolar amounts of 1 and 2, and the provided by a Presidential Young Investigator Award from the calculated ratio, ip(l)/i,(2) = 1.0, confirms the involvement of National Science Foundation (CHEM-8958027) and the Petroleum Research Fund is gratefully acknowledged. Partial funds (23) [(FII~P)~N~~[~~-]Q.~CH l2 (Cr.sH,,.CI,.~Fc,MnlNP,,P~): (I = 16.246 forequipping thesingle-crystal X-ray diffraction facility at UNC(10)A. b = 17.844 (6) f c = 17.991, (7) A,a = 81.38 (3)O.B = 69.62 C H were provided by a grant from the National Science (4)'. 7 = 71.34 (4)'. Data COlleclion on a Rigaku AFC6S diffraetometer yielded 2479 unique observed ( I t 2.5~(/)) reflections. A Foundation (CHE-8919288). disordered CH2C12 present in the lattice was modeled by three chlorine atom p i t i o n s with appropriate refined oeeupaneics. Due to the very low number of reflections Observed for the best crystal in a series of marginallydiffractingcryrtals. the phenyl groupon theeations and the Cp groups on the anion were modeled as rigid groups. Least-squarer refinement of 393 parameters converged at R (R.) = 0.1 I (0. I I ), and GOF = 2.48. (24) The calculationswcreperformed within the extended Hilekel formalism with use of the weighted H,,formula "ring program No. QCMP 01 1 (Quantum Chemistry Program Exchange. Indiana University). The atomic parameters were adopted from: Halct. J. F.: Hoffmann. R.: Saillard. J. Y.Inorc. Chcm. 1985. 24. 1695. (25) River. A. B.; Xiac-Zcng. Y.: Fcnrke, R. F. Inorg. Chcm. 1982.21.2286.
Supplementary Material Available: UV-vis spectra of 1. 2. and Z2-. an EPR spectrum of 2.. and text and tables giving complete crystallographicdata and resultsfor [PPN]*[2'-].0.6CHzClz(I3 pages). Ordering information is given on any current masthead page.
(26) When the potentials far removing the lint and second electrons are equal. a AEpvalue of 42 mV is predicted. The observed value AE 48 mV predicts that the potential for removal of thcsecond electron!i; 10 mV more positive than the potential far removal of the first. (27) ( a ) Ohst. H. H.: Kaehi. J. K. lnorc. Chem. 1986.25.2066. (b) Ohrt, H. H.; Kaehi. J. K. I . Am. Chem. Soe. 1986. 108. 2897.
